Delineating neurons that underlie complex behaviors is of fundamental interest. We will exploit a novel method for extremely rapid changes in excitability of genetically targeted neurons to affect a robust and vital ongoing regulatory behavior in rodents, i.e., breathing. Breathing is a remarkable behavior that mediates gas exchange to support metabolism and regulate pH. A reliable and robust rhythm is essential for breathing in mammals. Failure to maintain a normal breathing rhythm in humans suffering from sleep apnea, apnea of prematurity, congenital central hypoventilation syndrome, hyperventilation syndrome, Rett syndrome, and perhaps sudden infant death syndrome, leads to serious adverse health consequences, even death. Various neurodegenerative diseases, such as Parkinson's disease, multiple systems atrophy and amyotrophic lateral sclerosis, are associated with sleep disordered breathing that we hypothesize results from the loss of neurons in brain areas controlling respiration. If breathing is to be understood in normal and in pathological conditions, the mechanisms for respiratory rhythmogenesis must be revealed. We focus on two brain sites essential for generation of the normal breathing pattern, the preBvtzinger Complex and the retrotrapezoid nucleus/parafacial respiratory group. Using a viral delivery system, we will express genetically encoded opsins in various phenotypes of neurons in these key regions. Rapid changes in excitability of these neurons by administration of light pulses delivered via an optical fiber implanted in these sites in anesthetized, awake or sleeping rats should produce noticeable, even profound perturbations in breathing. Analysis of such perturbations will provide an extraordinary window into understanding mechanisms of respiratory rhythm and pattern generation.